A number of versions of such honeycomb bodies are known, for instance from Published European Application No. 0 245 737 B1 corresponding to U.S. Pat. Nos. 4,923,109 and 4,832,998; Published European Application No. 0 245 738 B1, corresponding to U.S. Pat. Nos. 4,803,189 and 4,946,822; or Published International Application WO 90/03220, corresponding to U.S. Pat. No. 5,135,794.
It is also known that for the most effective possible exhaust gas cleaning, it can be appropriate to place a succession of disks in the flow direction, and that these disks may optionally also have a different honeycomb structure or size from one another. However, such a body is no longer monolithic and therefore its production, its installation in a jacket tube, and optionally its coating all involve greater effort and expense. Such bodies made of more than one disk are described in Published International Application WO 90/04087, corresponding to U.S. application Ser. No. 805,097, filed Dec. 10, 1991; or Published European Application No. 0 121 175 B1, for instance. It is also known from U.S. Pat. No. 3,785,781 to place such disks in succession without them being spaced apart from one another.
In order to improve the effectiveness and/or the flow conditions in a monolithic honeycomb body, it has also already been proposed to offset the individual channels from one another or to interrupt them by making various structural provisions, such as in Published European Application No. 0 186 801 A2, corresponding to U.S. Pat. No. 4,665,051; Published European Application No. 0 152 560 A1; or German Published, Non-Prosecuted Application DE 29 02 779 A1, corresponding to U.S. Pat. No. 4,273,681. That creates additional edges facing into the flow of exhaust gas, which can be advantageous for catalytic conversion. A similar effect can also be achieved by folding-over some of the channel walls in cuff-like fashion, as is described, for instance, in German Petty Patent No. DE-U 89 09 128, corresponding to U.S. Pat. No. 5,045,403.
A common feature of the monolithic honeycomb bodies which are constructed in that way is that they have the same number of sheet-metal layers in each cross-sectional region, so that the catalytically active surface area remains the same in each cross-sectional region even if a comparable effect with respect to the leading edges is achieved by means of different sheet-metal structures as compared to that achieved if the number of channels per unit of surface area is increased. However, the catalytically active surface area per cross-sectional region cannot be changed in that way.
In monolithic honeycomb bodies, which are preferably used for the sake of ease of manipulation and installation in an exhaust system, the catalytically active surface area in a first cross-sectional region in the prior art is therefore the same as the surface area after it, in each cross-sectional region in the flow direction. That only allows limited optimization of the conditions in a monolithic honeycomb body in terms of response behavior and resistance to thermal aging. If it is desired for the honeycomb body to reach an adequate temperature for catalytic conversion as soon as possible upon cold starting of the engine, for instance, then it should not have overly large dimensions in its first cross-sectional region, yet in monoliths that necessarily means that it must keep the same dimensions in each succeeding cross-sectional region as well, so that for complete conversion the body has to be undesirably long (low number of channels and therefore long channel length).
However, if the body is constructed with many channels and a short structural length, then on one hand it has an unfavorable cold starting performance under some circumstances, yet after attaining operating temperature it already converts by far the greatest part of the pollutants in the exhaust gas in exothermic reactions in a forward cross-sectional region, so that a maximum temperature occurs in the first subsection and leads to premature aging of the catalytically active coating. The high temperature is imparted by the flow to the succeeding layers as well, thereby causing them to age thermally also, even though they make only a small contribution to the catalytic conversion.